To enter this challenging asana, start in adho
mukha svanasana, then step the legs together so the toes are
touching. Lift one leg off the ground and extend it upward without
twisting the hips. For a deeper expression of the pose, bend
the knee, dropping the ankle toward the buttocks.

Tri pada adho mukha svanasana is known in English as three-legged
downward-facing dog pose. The pose may
also be called ekapada adho mukha svanasana (one-legged downward-facing dog pose).

Yogapedia explains Tri Pada Adho Mukha Svanasana

Tri pada adho mukha svanasana is an advanced variation of one of yoga's most basic asanas, adho mukha svanasana, more commonly known as downward-facing dog. Practicing tri pada adho mukha svanasana improves balance, calms the mind and strengthens the shoulders and arms. It also opens the chest, hip flexors and hamstrings, while stretching the quadriceps, particularly in the full expression of the pose.